It's an accumulation of salts from your irrigation or possibly from soil additives. If you've seen your grannie's very old potted plants, they will have rusty yellow orange buildup, sometimes ivory at first, that is the same thing. You could pick this little bit off, but the real solution is perhaps too drastic for this time of year. Some plants you would submerse in water, and through osmosis, the salts would dissolve and transfer to the water until it reaches equilibrium with the soil: becomes isotonic. Then you drain the plant, continue to treat it as usual with the possibility of switching to rain water or distilled water or aquarium water to decrease any salts in your municipal water.
Some plants will not tolerate this flooding and you would have to repot with fresh soil.
This looks as if your plants will be fine for the rest of the summer. It's not a really bad buildup.